I really like this one pack i am thinking about changing (not primary pack now)... it was too small to use. I'd like to extend the neck for more capacity. i happen to also be in the beginning of a large PCT sectional and want to be light. I'd like to refit this other backpack to make that one my pct pack. Do you think joe from zpacks might be able to use material to add to / change / enlarge this other pack I am thinking about? I want to be as light as possible while comfortable and this effort is part of that. If joe can't do it then can anyone recommend another person I might talk to??

Im talking about basically replacing all the material besides the bottom, back and straps.

One possibility might be Chris Zimmer of Zimmerbuilt.com. Without seeing your actual pack its tough to know if it would work but I'm kinda skeptical. Changing out some but not all of the fabric would save some weight but maybe not as much as you think. A lot of a pack's weight is in the straps, belt and harness. You might be better off just buying a pack.